As is known, so-called "thermoforming" machines, fall into two quite separate classes depending upon their method of operation; firstly there are thermoforming machines which form and cut the sheet at separate workstations and secondly there are machines which form and cut the sheet at a single workstation. Such machines are used in the production of objects of thermoplastics material, e.g. "disposable" objects, from a sheet of hot pressed material.
This invention relates to a method and apparatus which falls into this second class, in that the shaping or forming of objects and their separation from the sheet of starting material is effected at a single workstation.
The state of the art for thermoforming methods and apparatus incorporating a single forming station, separation from the initial sheet and subsequent removal from the press, is represented by conventional single station machines and those according to U.S. Pat. No. 4,560,339 which will be briefly described hereinafter with reference to FIGS. 1 to 13 of the accompanying drawings, in which:
FIG. 1 is a side view in diagrammatic form of a thermoforming machine having an upper male die and a single lower female mould shown in the open press position prior to forming;
FIG. 2 is a side view of the machine of FIG. 1 shown with the press closed during the forming stage;
FIG. 3 is a view similar to that shown in FIG. 2 with the press closed during the cutting stage;
FIG. 3a is a schematic view of a detail of FIG. 3 shown on an enlarged scale;
FIG. 4 is a side view of the machine of FIG. 3 shown with the press open after the formed object has been cut out but still remains in the press;
FIG. 5 is a similar view to that in FIG. 4 but at a subsequent stage in which the pressed object is removed from the female mould and removed from the pressing area to a stacking collector;
FIG. 6 is a side view of a variation of the machine shown in FIG. 5 having a multiple row of moulds which discharge in bulk to a separate stacker;
FIGS. 7 to 10 show diagrammatic views of another variation of the machine shown in FIG. 5, in which the pressed objects are removed from the pressing area by means of a suction plate which can be placed between the male and female parts of the press;
FIG. 11 diagrammatically illustrates a plate which can lift a plurality of articles pressed in a single pressing operation by suction, used in the machine according to FIGS. 7 to 10;
FIG. 12 diagrammatically illustrates a front view of a thermoforming machine having a single upper male die and a double lower female mould which moves alternately beneath the male die; and
FIG. 13 includes a diagram which illustrates the stages and opening/closing cycle times for a thermoforming press having a single female mould.
The thermoforming cycle in machines which carry out forming and cutting at a single workstation can be described schematically as follows.
Stage 1: Sheet feed with the press open (FIG. 1).
A portion of sheet material A is delivered from a feed role (not shown) and is heated by irradiators B, e.g. infrared sources, and caused to advance between the open portions of the press, that is into the thermoforming area, in the direction of arrow F. The press consists of an upper male part or die M.sub.1 and a lower female part M.sub.2. Both parts M.sub.1 and M.sub.2 may be single or multiple, in which latter case several objects O can be obtained from each pressing operation.
Stage 2: Forming while the press is closed (FIG. 2).
The lower female part M.sub.2 rises against fixed male die M.sub.1 and object O is hot formed by causing rough shaping die M.sub.1p to enter the cavity M.sub.2c of female mould part M.sub.2 and thereafter injecting compressed air or applying negative pressure through a hole FO in male die M.sub.1. The compressed air causes the material A around rough forming die M.sub.1p to adhere to the side wall of cavity M.sub.2c and movable base M.sub.2f thereof. Then cavity M.sub.2c is cooled by a water cooling circuit generically indicated by C, and object O is also cooled and then subjected to a process of progressive structural and dimensional stabilisation.
Stage 3: Cutting (FIGS. 3 and 3a).
In this stage, all the lower part M.sub.2 of the press moves a distance ST, usually 1.5 mm (FIG. 3a) towards upper die M.sub.1, which is sufficient to separate the edge BO of object O from starting sheet material A.
The three stages described above are common to all thermoforming machines which shape and cut at a single workstation, or forming area (see diagram in FIG. 13). However a distinction is made between two classes of thermoforming machines on the basis of the different arrangements used to remove thermoformed objects O from the female mould M.sub.2 and subsequent operations (counting, stacking, etc.). The former have a single female mould M.sub.2, the latter has a double female mould, one on the left M.sub.2c and one on the right M.sub.2d, which are integral with each other, as will be explained below with reference to FIG. 12 and the object of Italian Patent No. 1,073,243.
The cutting stage (stage 3) completes the closed press cycle of operations (FIG. 13).
After this stage, the lower part M.sub.2 of the press descends, or moves away from upper part M.sub.1, following an S-shaped track (FIG. 4) carrying with it formed and cut object O, which therefore moves below the waste starting material A. Subsequently moving base component M.sub.2f of lower mould M.sub.2 ceases its downwards movement while the remaining part of mould M.sub.2 continues to move downward, thus drawing out and releasing object O completely from cavity M.sub.2c of mould M.sub.2.
At this point in the cycle there arises the difficult problem of moving or removing the thermoformed object or objects O from the forming area and stacking them. Various systems have been proposed, and the two most widely in use will be described below.
The first is the air blowing system (FIG. 5) which consists of supplying compressed air along a main CL provided with nozzles U which produce jets G which lift up objects O, causing them to tip slightly or topple into one or more collection channels R. The objects, continuing along collection channel R, become stacked forming a stack P.
This system can however only be used if the objects in mould M.sub.2 are arranged in a single file.
If the objects in mould M.sub.2 are in several rows (multiple files) the air jets G may remove objects O from the area bounded by parts M.sub.1 and M.sub.2 of the press, but they will be discharged in bulk. In order to get these back into order and then stack them, it is necessary to have a suitable stacking device PL (FIG. 6), which is separate from the thermoforming machine, but whose function must be synchronised therewith. This naturally involves heavy additional costs, greater complexity in operation and a high percentage of damaged objects which then have to be rejected.
In thermoforming machines provided with a stacker PL, objects O are "removed" by blowing them out of the forming area and causing them to impact against a stop surface AR before falling into a collecting bin V. At the base of collecting bin V there may be provided, as one of a number of possible systems, a conveyor belt collector TR controlled by a Maltese cross device which has a plurality of panels hinged together, each of which has a hole W.sub.1 which houses one object O. After impacting against surface AR which is orientated with a well-defined inclination, the objects end up by falling into holes W.sub.1 to be then transported intermittently towards a pusher I.sub.p controlled by a toggle E which pushes them one by one into a collection channel R where they form a stack P.
This system, which is mentioned here by way of example as one of many similar types, has many disadvantages.
To begin with, only circular objects which are taller than particular minimum dimensions can be stacked. It is not therefore possible to stack objects which are constrained to a specific orientation, e.g. objects of rectangular shape.
Also, in falling and being mixed together in bin V, many objects become deformed or otherwise damaged.
For these reasons the known system illustrated in FIG. 6, and others like it, are now considered to be obsolete.
The second system using a suction plate is illustrated diagrammatically in FIG. 7 to 11 and is the object of Italian Patent No. 1,175,178. During the opening T of the press, which is effected by lower female mould M.sub.2, a suction plate PA is inserted between the upper edge B of thermoformed object O, but blow waste A of the starting material, in such a position that object or objects O can be sucked up when mould M.sub.2 has finished its descent T (press completely open).
Mould M.sub.2 then stops in its lowest position to allow suction plate PA to move away from the area bounded by dies M.sub.1 and M.sub.2 (forming area--FIG. 8), carrying with its objects O to an area adjacent to the thermoforming machine where they are picked up by suckers VS hinged on arms BG of a stacker R (FIGS. 9 and 10). As soon as plate PA reaches the position illustrated in FIG. 8, and is therefore outside the forming area, mould M.sub.2 begins its upwards movement, sheet material A advances by a further step and a new thermoforming cycle is thus initiated. At the same time suckers VS (FIG. 9) takes objects O from plate PA. By means of a rotation of arm BG and a simultaneous rotation about pin Q suckers VS are turned over and carried against a multiple collector R where the objects are stacked into stacks P (FIG. 10).
The difficulties and disadvantages which limit the performance of the extraction system in thermoforming machines with a single female mould are:
1. The very long distance T by which the press must open, which is the sum of the height of the object or objects O and the distance travelled S (FIGS. 4 and 7) plus a specific distance to ensure a reasonable margin of safety (FIG. 7). However, as the time used in moving distance T is time which is of no use to the cycle it will be understood that, other things being equal, the output of the machine will be lower the taller the objects O which have to be removed.
2. Travel S is provided to create the space necessary for plate PA to insert itself between material A and thermoformed object or objects O. Keeping travel distance S short implies creating problems with the size of suction plate PA. Increasing distance S means appreciably lowering the productivity of the thermoforming machine. The definition of track S and the dimensioning of plate PA are therefore always the result of a compromise. However the dimensions of suction plate PA are also governed by the requirement that it must not impede the advance of sheet material A, which being in a heated state, has a tendency to weaken and fall downwards.
3. Keeping the press open for the time required for suction plate PA to enter and leave the forming zone increases the dead time in the thermoforming cycle (FIGS. 7, 8 and 13).
The fact that presses have to stand open for the time required for inserting plate PA between material A and object or objects O which have to be removed, sucking up the objects and removing them (a time which is commonly equal to 30% of the cycle time, i.e. a by no means negligible period) has a greatly adverse effect on the productivity of the machine.
If relatively large objects or small objects in a number of rows are present in mould M.sub.2 then the time for moving plate PA increases, because the distance T which has to be travelled is greater, or because plate PA has to travel a back and forth distance which is at least equal to the width LA of mould M.sub.2 (FIG. 7).
4. The time for which object or objects O remain n the press, that is with their own walls in contact with the cooled wall of cavity of M.sub.2c of mould M.sub.2, is another important parameter which affects the productivity of a thermoforming machine. In fact as soon as mould M.sub.2 reaches the position illustrated in FIG. 4, object O is displaced away from the wall of cavity M.sub.2c and therefore the stabilising cooling of the plastics material which has been subjected to thermoforming ceases. As may be seen from the diagram in FIG. 13, the cooling time tst for stabilisation in the press is the time between the injection of the forming air t.sub.1 (through hole FO) and the start of removal t.sub.3 (FIG. 4). Normally this time is equivalent to about half the cycle time. When it is necessary to increase cooling time tst the rate of the thermoforming machine has to be slowed.
5. Because the objects are directly sent to the stacker from plate PA it is not possible to carry out any additional operations (e.g. perforation, labelling or the like) on objects O between the removal operation and stacking.
If it is necessary to carry out additional operations on thermoformed and stacked objects O, the objects have to be repositioned, and this usually means destacking them and sending them to suitable machines to carry out the required operations, with a consequent risk of damaging the objects and producing rejects.
One of the greatest disadvantages of thermoforming machines with suction plate extraction lies in the shape and restricted dimensions of the suction plate itself. In fact, with reference to FIG. 11, it will be noted how the pressure difference, between the external environment and the negative pressure created by the suction through plate PA in the space within thermoformed objects O, creates the force by which objects O are attracted to and held against the plate. It can be said to a good approximation that this pressure difference is equal to the loss of head which air flows Q1/2 and Q2/2 and Q3/2 create when passing through slots FE along edges BO.
Within Plate PA then:
the air in section S.sub.1 will have a velocity V.sub.1 which will cause a flow Q1 to pass, PA1 in section S.sub.2 it will have a speed V.sub.2 which is different from V.sub.1 and such as to pass a flow Q1=Q2. PA1 in section S.sub.3 the speed will be V.sub.3 which is different from V.sub.1 and V.sub.2 and such as to cause the passage of a flow equal to Q1+Q2+Q3. Ideal operating conditions arise when Q1=Q2=Q3. As suction plate PA1 forming, for example in M.sub.1 -M.sub.2s PA1 cutting out PA1 press opening PA1 lateral movement of the double female mould PA1 closing of dies M.sub.1 -M.sub.2d PA1 forming in M.sub.1 -M.sub.2d PA1 the production of a pressed object by hot forming and cutting out of the object or objects between one half of a movable double female mould which is located in the forming area opposite a male die, PA1 Movement of the female half of the mould, said half carrying the thermoformed object or objects therewithin, towards a corresponding discharge area alternately to one side or the other of the forming area, PA1 removing the object or objects from the female half of the mould in its corresponding discharge area and transferring each object to one of a plurality of receiving formers of a shape corresponding to the mould, which formers can move sequentially in steps along a track, PA1 sequential movement of the formers towards least one workstation or handling station at the same time as the female mould moves back to bring its other half opposite the male die in order to produce a subsequent formed object, and PA1 carrying out at least one processing or handling operation on all the objects carried by at least one former at the same time as the next formed object is produced. PA1 a male die and a double female mould, one half of which mould can move alternately with respect to the other half for mating with the male die in turn while at the same time carrying the object or objects produced previously in the other half of the female mould to an easily accessible discharge area, PA1 a sheet feed to deliver a sheet of thermoforming material in steps between the male and female moulds, PA1 a cutting device which is brought into operation at the end of each closing movement of the press, PA1 a stepwise conveyor having a plurality of plates or formers, each of which is capable of receiving and supporting the object or objects from one forming operation in the same mutual positions which they occupied in the press, PA1 at least one pick-up head capable of lifting one set of formed objects alternately from each half of the double female mould from one side or the other of the male die and of transferring them to a corresponding plate or former on the conveyor and PA1 at least one processing or handling station located along the conveyor for the simultaneous handling or processing of all the objects from at least one forming operation.
has to be inserted between sheet material A and the upper edges BO of objects O, its height dimension must be as small as possible so as not to constrain the distance by which the press opens and make it excessively long. In practice therefore it is preferable to use a configuration in which V.sub.3 is very much greater than V.sub.1, so that when it is in operation Qt is greater than Q2, which is in turn greater than Q3. This means that the system operates under conditions which are very far from optimum conditions.
In thermoforming machines which have double lower female mould of the type disclosed in Italian Patent No. 1,053,243 and illustrated diagrammatically in FIG. 12, the objects are formed by alternately coupling the two female parts M.sub.2s and M.sub.2d with the single upper male part M.sub.1.
When dies M.sub.1 and M.sub.2 are closed the thermoforming takes place in the manner described above in stages 1 and 3.
With such machines it is possible to obtain advantages in subsequent stages, in that:
1) The distance T travelled by the press on opening its independent of the depth or height of the thermoformed objects and may therefore be kept at a minimum value sufficient to feed in material A, with a great reduction in the passive time in the cycle, as is shown in the diagram in FIG. 13.
2) The press remains open only for the time necessary to advance sheet material A and effect the alternate lateral movement of female moulds M.sub.2s and M.sub.2d. These operations take place at the same time, without providing any stoppage for removing thermoformed objects O.
3) The time for which objects O remain in cavity M.sub.2c with their walls in contact with the mould is longer than the thermoforming cycle (FIG. 13), because objects O remain in close contact with the die from the time of cutting until the next thermoforming operation in the other female mould. In other words the cooling of a thermoformed object O is extended throughout the following stages:
The stacking of objects O on these thermoforming machines takes place in stacker R.sub.s in the case of the objects formed in M.sub.2s during forming stage in mould M.sub.2s and R.sub.d in the case of those formed in M.sub.2d.
Base members M.sub.2fs and M.sub.2fd remove objects O by travelling distance C.sub.t (FIG. 12) and stack them alternately pushing the objects formed in female mould M.sub.2s into left hand collection or stacking device R.sub.s and those formed in female mould M.sub.2d into right hand collection or stacking device R.sub.d. Devices R.sub.s and R.sub.d are located to the sides of the fixed part of die M.sub.1. It may happen therefore that objects O which have recessed angular supporting portions (e.g. 2, 3, 4 or more indentations or indented feet located at the same level) provided, as is usual in the art, to prevent one object binding completely within another during stacking, (which would make it impossible for the objects to be then destacked for use), are located in a perfect vertical orientation and alignment, because they all come from the same mould. In this case it will be seen that there is accurate superimposition of two or more consecutive stacked objects, so that the spacing effect of the feet or indentations is neutralised and as a result the objects bind firmly together making it difficult to separate them.
Another cause of poor spacing and therefore of irreversible binding between the stacked thermoformed objects lies in inaccurate forming of the stacking feet or indentations (which generally project towards the interior of the object by a fraction of a millimeter or a little more), as, being made of a thermoplastics material which is therefore dimensionally unstable at the time of forming, these are obtained with fairly wide dimensional tolerances. There will therefore be variations in the connecting angle to both the feet and the base of each object, depending on the nature of the thermoplastics material used, and the shape and depth of the objects being thermoformed, which are unacceptable for correct stacking of the objects.
Objects which do not destack, or which destack with difficulty, require manual intervention, reduce productivity and almost always end up by increasing the number of rejects.
As will be noted, the distance C.sub.t travelled by base members M.sub.2fs and M.sub.2fd depends on the maximum height of the objects O being formed and is therefore longer when the objects are deeper. It is essential in fact that interference between the bottom of object O and the lateral movement and closing of the press is avoided.
FIG. 12 illustrates pins S.sub.p which centre parts M.sub.2s and M.sub.2d with respect to fixed part M.sub.1 as the press closes, through being inserted by a certain amount into corresponding seats Z provided in mould M.sub.2. A.sub.1 also indicates diagrammatically the members which feed or advance sheet material A.
As in thermoforming machines having a single female mould, so in machines having a double female mould, stacking of the objects takes place without there being any practical possibility of carrying out additional operations on thermoformed objects O, as these are gathered into stacks immediately after thermoforming.